J . Am. Chem. SOC.1990, 112, 8458-8465
8458
Synthesis and Characterization of Two-Directional Cascade Molecules and Formation of Aqueous Gels'" George R. Newkome,*.IbGregory R. Baker,lb Sadao Arai,lbVcMary Jane Saunders,ld Paul S. RUSSO,~~ Kevin J. Theriot,IbCharles N. Moorefield,lb L. Edward Rogers,lb Joseph E. T. Richard Lieux,le.f Mary E. Murray,lbqrBuffie Phillips,'b*fand Laura Pascallb.f Contribution from the Departments of Chemistry and Biology, University of South Florida, Tampa, Florida 33620, and Louisiana State University, Baton Rouge, Louisiana 70803. Received April 18, I990
Abstract: Numerous two-directionalcascade molecules (arborols) have been prepared via a simple two-step procedure. These cascade molecules have been abbreviated as [m]-n-[m] arborols, where m is the number of hydroxyl moieties on the cascade spherical surface and n is the number of methylenes connecting the cascade spheres. Several of the arborols in the [9]-n-[9] and [6]-n-[6] series form thermally reversible aqueous gels; properties of these gels are discussed. An aggregation model is proposed for the gel-forming molecules; preliminary molecular modeling calculations and electron micrograph data support this model.
Introduction The synthesis of the model arborol l 2provided the foundation for the synthesis of other cascade molecules. We consider arborol 1 to be a "one-directional" cascade polymer in that it possesses a central "trunk" (core) attached to a single cascade sphere. An obvious extension of this concept was the attachment of additional cascade spheres to the hydrocarbon backbone (or central core).
\ 1
Addition of a second sphere should affect the aggregation of the arborols by changing the overall topology; attachment of additional spheres may eventually lead to a unimolecular micelle, i.e. a highly branched, specifically shaped structure with an aggregation number of one. The attachment, or construction, of cascade spheres onto each end of a hydrocarbon chain forms a rwo-directional arborol. In following an organic "tree" a n a l ~ g ythe , ~ lipophilic core of these two-directional molecules should solubilize hydrocarbons in a n aqueous environment. The chemical and physical properties of these arborols will depend upon several factors: the length of the hydrocarbon backbone, the incorporation of functionality along the backbone, the flexibility of the backbone, the size of the cascade spheres, and the size, shape, and functionality of the organic species itself. For example, an arborol possessing a central ( I ) (a) Cascade Molecules Part 6. For Part 5, see: Newkome. G.R.; Moorefield, C. N.; Theriot, K. J. J . Org. Chem. 1988, 53, 5552. (b) Department of Chemistry. University of South Florida. (c) Visiting scholar from the Tokyo Metropolitan University.Tokyo, Japan. (d) Department of Biology, University of South Florida. (e) Louisiana State University. (f) Undergraduate. (2) Newkome, G. R.; Yao, 2.-Q.;Baker, G. R.;Gupta, V. K. 1. Org. Chem. 1985, 50, 2003. (3) (a) HallC, F.; Oldeman, R. A. A.; Tomlinson. P. B. Tropical Trees and Foresrs: An Archirerrural Analysis; Springer-Verlag: Berlin, 1982;Am. Scienrisr 1983, 71, 141. (b) Wilson, B. F. The Growing Tree; University of Massachusetts Press: Amherst, 1984.
unsaturated moiety should permit chemical modification within the lipophilic region. Construction of the arborol with a catalytic moiety appended to the lipophilic backbone should provide an enzymatic model where catalysis occurs at a lipophilic site within a hydrophilic e n ~ i r o n m e n t . ~ The synthesis of a series of two-directional arborols was undertaken to explore their properties, and herein we report the results. Attachment of the molecular spheres was shortened* to a two-step procedure: reaction of an a,w-dibromoalkane with either ethyl sodiomethanetricarboxylateor methyl sodiomalonate, followed by amidation with tris(hydroxymethy1)aminomethane ("Tris"). This shortened procedure gave high overall yields while providing sufficient surface hydroxyl moieties per sphere to impart the desired water solubility. These spheres are not as highly branched or extended as those of arborol 1, or those of Tomalia and co-workers;s however, CPK molecular models show that each terminal sphere has a radius of ca. 12 A. W e have abbreviated the nomenclature for this family of cascade molecules as [m]-n-[m] arborols, where m designates the number of terminal groups for each sphere and n denotes the length of the hydrocarbon bridge. The hydrocarbon (lipaphilic) backbone connecting the two spheres generates an inner region, which should be highly compressed in aqueous solution, in the absence of aggregation (see Figure I). This concept was verified by molecular modeling6 of a [9]-10-[9] arborol. Several of the arborols formed thermally reversible aqueous gels ([9]-n-[9]: 10 5 n -< 13; [6]-n-[6]: 8 5 n I 13), which were characterized via optical and electron microscopy, and light scattering techniques.
Synthetic Aspects The first series of two-directional arborols was prepared by a two-step nucleophilic substitution-amidation procedure. The hexaethyl esters 2a-k were prepared ( 7 2 4 2 % ) by the reaction of the appropriate a,w-dibromoalkane with NaC(C02Et)3 in a C6H6-DMF (1:1) solvent mixture a t 90 OC for 24 h; shorter reaction times resulted in the recovery of some unchanged starting material. Because of the essentially hydrocarbon backbone of 2a-k, the 'H NMR spectra were quite simple: a triplet (6 I .21, J = 7.1 Hz)and a quartet (6 4.20, J = 7.1 Hz)for the ester methyl and methylene, respectively, and a broad singlet ( 6 1 S O ) for the (4)Menger, F. M. Topics Curr. Chem. 1986, 136, 1. ( 5 ) Tomalia, D. A.; Hall, M.; Hedstrand, D. M. J . Am. Chem. Soc. 1987, 109, 1601. Tomalia, D. A.; Berry, V.; Hall, M.; Hedstrand, D. M. Macromolecules 1987, 20, I164 and references cited therein. (6) Preliminary calculations using QuANTA/cHARMm: Polygen Corpora-
tion, 200 Fifth Avenue, Waltham, MA 02254. For a description of the CHARMm force field, see: Brooks, B. R.;Bruccoleri, R. E.; Olafson, B. D.; States, D. J.: Swaminathan, S.; Karplus. M. J . Compuf. Chem. 1983, 4, 187-21 7.
0002-7863/90/1512-8458$02.50/00 1990 American Chemical Society
J . Am. Chem. SOC., Vol. 112, No. 23, 1990 8459
Two- Directional Cascade Molecules and Aqueous Gels Scheme I"
CONH
B r - ( CH2 ) -B r n i l , .
l9l-n-IQl
..,
-
Me%$-(CH2)nke Me0
HHO
5 HNOC
CONH
Me O
4
0
5
Figure 1 . Topology of the two-directional arborols: (a) extended and (b) compressed conformations.
bridging methylenes. The infrared spectra of these hexaesters are all similar, characterized by a C = O stretch at ca. 1740 cm-I and C-0 stretches at 1267 and 1223 cm-'. The I3C N M R spectra confirmed the proposed structures of 2a-k, since unique resonances were obtained for each of the carbons. The triester moiety has four characteristic peaks: ca. 6 14 for the OCH,CH,; ca. 6 62 for the OCH2CH3;ca. 6 65.8 for the Cquat; and ca. 6 168 for the C=O. The methylene adjacent to the triester has a chemical shift of ca. 6 33.5. The bridging methylene carbons have chemical shifts in agreement with empirical va 1ues .'** Hexaethyl esters 2a-k were converted to the desired [9]-n-[9] arborols 3a-k by reaction with Tris in M e 2 S 0 at 70 "C via the previously described procedure (Scheme I);2 however, purification of the products proved extremely difficult. Similar solubility properties of the [9]-n-[9] arborols and Tris made separation by precipitation impossible; thus, a modification of reaction conditions was deemed appropriate and several variations were investigated. On the basis of the results of Prelicz et al,9 reaction of 2d with Tris neat at 1 20-1 40 "C was attempted; however, only a paucity of 3d was obtained with the remainder of the material as intractable brown solid. The next variation was the attempted amidation of hexaester 2d in a suitable solvent without added base; M e 2 S 0 was again chosen since it solubilized both reagents (hexaester and Tris) and allowed reaction temperatures above 100 "C. When a M e 2 S 0 solution of hexaester 2d and Tris was stirred for 15 h at 25 "C, (7) Levy, G. C.; Nelson, G. L. Carbon-13 Nuclear Magnetic Resonance for Organic Chemists; Wiley-Interscience: New York, 1972. (8) Breitmaier, E.; Voelter, W. Carbon-13 N M R Spectroscopy, 3rd ed.; VCH: New York, 1987. (9) Prelicz, D.;Sucharda-Sobczyk, A.; Kolodziejczyk, A. Rocz. Chem. 1970, 44, 49; Chem. Abstr. 1970, 73, 2 4 8 7 3 ~ .
unreacted ester 2d and amine were recovered (>90%), as determined from the 13CNMR spectra of the CH2C12and water soluble components, respectively. Similar results were obtained when this reaction was repeated at higher temperatures (up to 130 "C); however, addition of anhydrous K2C03to the reaction at 130 "C caused a color change (clear to light yellow) in approximately 30 min. N M R analysis of the crude product mixture showed the presence of the desired arborol3d, some unreacted Tris, and no trace of unreacted ester. Decreasing the reaction temperature to 25 "C and using an exact 6:l molar ratio of Tris to ester afforded (88-93%) the [9]-n-[9] arborols, which were free of unreacted Tris. Reaction of Tris with these triesters has been erratic, and sometimes results in extensive decarbalkoxylation to give the corresponding [6]-n-[6] arborol and several byproducts. The 'H N M R spectra of 3 were quite simple, each consisting of a broad singlet (6 1.5) for bridging methylenes, a broad singlet (6 -3.7) for exterior hydroxymethylenes, and a broad singlet (6 -5.2) for amide hydrogen. Complete transformation from 2 to 3 was verified by 13C N M R spectroscopy: an 8 ppm downfield shift of C=O (2 (6 167) to 3 (6 175)), disappearance of the OCH2CH3resonances (2 (6 62.2 and 14.0, respectively)), and appearance of N H C and C H 2 0 H resonances (3 (6 64.8 and 63.2, respectively)). Acceptable elemental analyses of 3 proved difficult partly due to their hygroscopicity; attempted drying (70 "C, 1 mm) did not improve the results. Thus, the purities of 3 were established by the absence of extraneous peaks in the I3C N M R spectra (>95%) and reversed-phase HPLC (>97%). Further proof of their structures was obtained by preparation of the acetate derivatives via standard conditions;1° the [9]-n-[9] arborols (3) decarboxamidate under the reaction conditions to give an acetate identical (13C N M R spectra) with that formed by the corresponding [6]-n-[6] arborol (5). The 13CN M R spectra have signals at ca. 6 20.5 for COCH,, ca. 6 55.7 for CH, ca. 6 57.7 for NHC, ca. 6 62.0 for CH20Ac, and ca. 6 170.2 and 171.2 for C 0 2 and CONH, respectively. The IR spectra of arborols 3 are practically superimposable; there is a broad 0-H stretch at ca. 3320 cm-I and C-H stretches at 2953, 2924,2855 cm-'. The presence of the amide moiety was indicated by amide I (1 65 1 cm-') and amide I1 ( 1628 cm-') bands and the absence of an ester carbonyl stretch (1740 cm-I). The ultraviolet spectra of these arborols show only one absorption (e.g., 3h X = 212 nm, t = 4674 L mol-' cm-I). Attempted melting point determinations for the [9]-n-[9] arborols 3 resulted in effervescent decomposition over a broad temperature range (ca. 120-1 60 "C). Heating the arborols at these temperatures probably causes ox-
-
-
~
(10) The arborol (0.2 mmol) was added to acetic anhydride (5.0 mL) and pyridine (0.5 mL) and then warmed to 50 OC for 15 h. The mixture was concentrated in vacuo to give a thick oil, which was purified by HPLC (ODS, MeCN) to give the acetate derivative.
Newkome et al.
8460 J. Am. Chem. Soc.. Vol. 112, NO.23, 1990 azoIine formation via loss of water;" oxazoline formation will be discussed in more detail later. All [9]-n-[9] arborols are slightly hygroscopic, water soluble solids and their aqueous solutions foam upon agitation. From this series, the [9]-n-[9] arborols with connecting hydrocarbon chains of 10 or more carbons (10 5 n 5 13) have the distinction of forming thermally reversible, thixotropicI2 aqueous gels'3 at concentrations as low as 1.0 wt 5%. The gel formed by [9]-IO-[9] arborol (3h) was subsequently studied to elucidate its structural properties. The analogous series of [6]-n-[6] arborols (5) was prepared to investigate the relationship between cascade sphere size, linkage distance, and gel formation. The tetraesters 4 were prepared by the reaction of a a,o-dibromoalkane with an excess of dimethyl malonate in DMF with K2C03. The IH NMR spectra of 4 consist of a broad singlet (ca. 6 1.3) for the central backbone methylenes, a multiplet (ca. 6 1.9) for the methylene adjacent to the malonate methine, a triplet (ca. 6 3.35) for the malonate methine, and a singlet (ca. 6 3.73) for the methyl ester. The infrared spectra of the tetraesters are characterized by a C S O stretch at 1757 and 1736 cm-' and C-0 stretches at 1202 and 1 157 cm-I. The I3C N M R spectra verified the structure of 4: an upfield shift of the substituted methylene [CH2Br, ca. 6 33; CH2CH(C02Me)2,ca. 6 291 and appearance of the three peaks characteristic of the methyl malonate moiety (6 ca. 51.5, 52.5, and 170.2 for the methine, methyl ester, and carbonyl, respectively). The peaks for the remaining methylenes had chemical shifts in agreement with predicted values.'J The reaction of 1 ,Zdibromoethanewith excess methyl malonate gave only the expected methyl I , 1-cyclopr~panedicarboxylate,'~ which arises via dialkylation. Similarly, the reaction of 1,4-dibromobutane with excess methyl malonate gave dimethyl 1 ,Icyclopentanedicarboxylateand a paucity (7%) of the desired 4c. Tetraesters 4 were converted to the [6]-n-[6] arborols 5 by the procedure described for [9]-n-[9] arborols. The 'H NMR spectra of 5 consisted of two broad singlets (ca. 6 1.15 and 1.60) for bridging methylenes, a broad singlet (ca. 6 3.54) for hydroxymethylenes, and a broad singlet (ca. 6 7.41) for amide hydrogen. I3C NMR verified the complete amidation of the tetraester: there was a I .O ppm downfield shift of C=O (4 (6 170) to 5 (& 171)), disappearance of OCH3 resonances (4 (ca. 6 52.5)), and appearance of NHCand CHIOH resonances (5 (ca. 6 62.2 and 60.5, respectively)). The IR spectra of arborols 5 are all similar: a broad &H stretch at 3312 cm-' and C-H stretches at 2949,2926, and 2857 cm-'. The amide moiety was confirmed by the presence of amide bands (1655 and 1632 cm-I). As with 3, attempted melting point determinations of 5 resulted in effervescent decomposition at temperatures of 120-160 OC. Gelation was also observed with 5, but with shorter hydrocarbon chains (8 I n 5 13) than observed with 3. The corresponding acetate derivatives were prepared and shown to be spectrally identical with samples prepared from 3, and also afforded correct elemental analyses. Three representative [3]-n-[3] arborols were prepared to evaluate the relationship between cascade sphere size and gel formation at the lower limit of cascade sphere size. Dimethyl pimelate and dimethyl dodecane- I , 12-dioate were prepared from the corresponding diacids via Fischer esterification and gave physical and spectral properties in agreement with literature data. These diesters and dimethyl glutarate were treated with Tris via the standard conditions to give the bisamides 6. The IH NMR spectra of 6 consisted of upfield signals indicative of the hydrocarbon backbone and downfield singlets at ca. 6 3.5, 4.75, and 7.0 for the hydroxymethylene, hydroxyl, and amide hydrogens, respectively. I3C NMR spectra gave peaks for each unique carbon; signals characteristic of the amide functionalities were 6 35.0 for ( I I ) Nys. J.; Libeer, J. Bull. Soc. Chim. Bel. 1956.65, 377; Chem. Absrr. 1957, 51, 10494d. (12) Lenk, R. S.Polymer Rheology; Applied Science: London, 1978; pp t -m *". (13) Ferry, J. D. Viscoelastic Properties of Polymers; Wiley: New York, 1970 pp 557-570. (14) White, D. A. Synrh. Commun. 1977, 7 , 559.
.
Scheme 11. Amidation of an Ester by Tris
-
ro HO
OH
'0H
CH2CONH, & 60.7 for CH20H, 6 62.0 for C and 6 174.0 for C=O. The infrared spectra also confirmed t%amide functionality: ca. 1630 cm-' (amide I), ca. 1570 cm-' (amide II), and absence of an ester carbonyl stretch at 1740 cm-I. Unfortunately, these bisamides showed very little or no water solubility, The generality of these amidation conditions was examined via reaction of hexaester with several other amines. The hexaester 2f (n = 8) was treated with tert-butylamine (7a), 2-amino-lethanol (Ib), 2-amino-2-methyl-1-propanol (7c), and 2-amino2-methyl-I ,3-propanediol (7d), employing the same conditions used for Tris. There are three possible scenerios: no reaction, amide formation, or triester monodecarbalkoxylation,which is a major side reaction with triesters.I5 The crude products from each reaction were analyzed by I3C NMR spectroscopy; the Occurrence of amidation or decarbalkoxylation was discerned by comparison with the I3C N M R spectra of 2f, 3f,4g, and 5g. Reaction of tert-butylamine (7s) with hexaester 2f gave an oil, whose 13C N M R spectra were identical with starting hexaester Zf with no indication of either decarbalkoxylation or amidation. The reaction of 2f with aminoethanol7b gave an oil, whose 13CN M R spectrum indicated extensive monodecarbalkoxylation. The reaction of 2f with aminopropanol7c gave an oil with a small amount of decarbalkoxylation. The I3C N M R spectrum of the material from the reaction with aminodiol7d indicated no hexaester 2f or malonic ester moieties; purification via HPLC gave two major products 8 and 9. EtOzC
COZEt
E~Ozc+(CH*)8+cozE~ EtOzC
"ZE1
w{2
OH
wo,
K&*
25 %, 15 h
9
Reactions of several esters with H,NC(CHzCH2CH20H)3 [bis(h~motris)]'~.'~ failed to give the desired amides under these reaction conditions; 13C NMR spectral data indicated the presence of partially transesterified bis(homotris). However, the desired amide was obtained by use of NaOEt in Me2SO; details will be presented in a future publication. Amidation of esters usually proceeds by the BAc2 mechanism" and is general base where a second molecule of the aminez0(or other suitable base) accepts a proton in the rate-de(IS) Newkome, G. R.; Baker, G. R. Org. Prep. Proc. Inr. 1986.18, 119. (16) Available from Aldrich Chemical Co.: 36,154-2. ( 1 7) March, J . Advanced Organic Chemisrry: Reacfionr. Mechanisms, and Srrucrure, 3rd ed.; Wiley-Interscience: New York, 1985; pp 226-227; 375-376. (18) Bunnett. J. F.; Davis, G. T. J . Am. Chem. Sac. 1960,82,665. Bruice, T. C.; Donzel, A.; Huffman, R. W.; Butler, A. R. J . Am. Chem. Sac.1967, 89, 2106. (19) Jencks, W. P.; Carriuolo, J. J . Am. Chem. Sac.1%0,82,675. Bruice, T. C.; Mayahi, M. F. J . Am. Chem. Soc. 1960,82, 3067.
J . Am. Chem. SOC.,Vol. 112, No. 23, 1990 8461
Two- Directional Cascade Molecules and Aqueous Gels
Figure 3. Negatively stained transmission electron micrograph of the [9]-10-[9] arborol (3h) gel. Bar = 300 A. Diameter of individual aggregate strands is ca. 35 A.
-
H r'w\
I
\I
nu
11
i
k ____________________.__________..__ 2 Q _._.___._..______________ 4 (A) of the [9]-10-[9] arborol (3h).
Figure 4. CPK dimensions Figure 2. Birefringence of the [9]-10-[9] arborol (3h) gel: (a) viewed between cross-polarized filters; (b) viewed between plane polarized filters. Bar = 100 pm.
termining step. The need for added base (anhydrous K2C03)could result from the steric bulk of both ester and attacking amine, which precludes proton abstraction by a second amine molecule. The reactions of these P-aminoalcohols reveal several mechanistic aspects of the reaction of an ester and Tris: initial N attack of the carbonyl is unlikely, since tert-butylamine did not react with 2f, under similar conditions; the observed decarbalkoxylations probably arise from initial 0 attack of the carbonyl (note: reactions with 7b and 7c); decarbalkoxylation was a predominate side reaction; and little or no amidation was observed unless the aminoalcohol possessed a second hydroxyl. This suggests that Tris reacts via the mechanism shown in Scheme 11, which proceeds by formation of the alkoxide of Tris; subsequent transesterification with the triester (or decarbalkoxylation); and amide formation (or decarbalkoxylation) via an intramolecular rearrangement. Such an intramolecular process should be rather facile. Similar reactions were previously observed by Nys and Libeer." Azeotropic removal of water from a refluxing xylene solution of acetate 10 gave a paucity of oxazoline 11; related reactions were also reported.
1eqAcOH
H2NC(CH20H)3
85 "c
CH20C0CH3
H2NfCH20H CHZOH
10
xylene A.3h
\OH 11
Characteriztion of the Gels The thermally reversible aqueous gels formed by [9]-10-[9] arborol (3h) and [6]-10-[6] arborol (5i)were studied by several techniques: viscometry, optical and electron microscopy, and light (20) Blackburn, G. M.; Jencks, W. P. J . Am. Chem. SOC.1968,90,2638. Bruice, T. C.; Felton, S. M. J . Am. Chem. SOC.1969, 91, 2799. Felton, S. M.; Bruice, T. C. J . Am. Chem. SOC.1969, 91, 6721.
scattering. In general, gelation was observed at concentrations of 2-10 wt % and the solutions exhibited pseudoplastic behavior during gelation. Arborols 3h and 5k also gelled in MeOH-H20 mixtures (up to a 4:l ratio); these gels appeared much stiffer than their aqueous counterparts. Since by definition a gel has no zero shear flow,13i.e. it exhibits no steady flow under conditions of zero shear, viscosity is an appropriate experimental method. The viscosity of an aqueous solution of 3h was measured during gelation. A 5.0 wt 5% aqueous solution of 3h was prepared at 80 "C, transferred to a viscometer at 25 OC,and the viscosity was measured (at various shear rates) as the solution gelled. At the onset of gelation the viscosity showed pseudoplastic behavior and then quickly increased as the solution gelled. The elapsed time was approximately 45 min. Optical microscopy of gelled solutions of 3h was investigated to detect the presence of any macroscopic structure. A gelled solution of arborol3h was lyophilized and observed through crossed polarizing filters; the dried gel showed no discernable structure. However, a sample of the original gel exhibited birefringence, indicative of an extended ordered structure or multiple scattering. Addition of water to the aqueous gel caused dilution of the sample until only limited areas of birefringence were visible (Figure 2). Further dilution of the sample led to complete disappearance of birefringence. The relatively small size and low molecular weight (FW = 1052)of 3h compared to a typical polymer ge1I3 implies probable aggregation of 3h into a larger structure prior to or during gelation. In order to discern the aggregate size and shape, the gel was negatively stained and examined by transmission electron microscopy. The long fibrous rod structure of the aggregates is clearly visible in Figure 3. These rods have uniform diameters (34-36 A) and variable lengths (ca. >2000 A). The dimensions of a single molecule of 3h were determined by examination of CPK space filling models and molecular modeling (Figure 4). The most interesting dimension of Figure 4 is the end-to-end distance of ca. 29 A, which compares favorably with the observed rod diameter of 34-36 A. The slight difference between the measured rod diameter and the end-to-end distance of 3h can be attributed to some degree of hydration of the cylindrical aggregate surface and the thickness of the stain used for the transmission electron microscopy. The agreement between end-to-end distance and rod
Newkome et al.
8462 J . Am. Chem. SOC.,Vol. I 12, No. 23, 1990
Figure 6. Fluorescence of chlortetracycline in the presence of the arborol
gel (3h). Table 11. pH Stability of the Arborol Gels
gelation* PHb
solution
3h
Si
yes yes HCl(0.25 M)commercial buffer' yes yes yes yes H3BO3 (0.1 M) yesd H3B03(0.75 M) H3BO3 (0.1 M) and NaCl (0.1 M) yes commercial buffer' yes yes 7.0 no no K2B407 (0.1 M) 9.2 yes yes K2B407 (0.01 M) yes yes K2B407 (0.001 M) 9.9 K2C03 and H3B03 yesd 10.0 commercial buffer' no yes 10.2 NH40H (0.25 M) yes yes 11.5 yes yes K2C03 (0.25 M) 11.6 KOH (0.25 M) yes yes 11.6 NaOH (0.25 M) yes yes "The arborol (20 mg) was dissolved in the solution (0.4 mL) at 80-90 OC, cooled to 25 OC, and allowed to gel. bpH was measured by a Corning Model 7 pH meter with a combination electrode. 'The pH 4.0 and 7.0 buffers were obtained from Mallinckrodt Inc. (composition unknown); the pH 10.0 buffer obtained from Fisher Scientific contained KOH, K2C03,and K2B407in unknown amounts. dVery slowly formed a soft gel. 2.1 4.0 4.4
Figure 5. Proposed aggregation of the [9]-10-[9] arborol (3h). Table I. Phase-Transition Temperatures (Gel to Solution) for Aqueous Solutions of [9]-10-[9] Arborol (3h)
concentration, wt % 2.12
temperature (g
4.34 6.22
48-49 54-55 65-66
8.15
69-70
-
s), "C
diameter suggests that molecules of 3h pack in a cross-over fashion (Figure 5) where the dumbbell-shaped molecules form an interlocking network. This arrangement maximizes intermolecular lipophilic as well as hydrophilic interactions and generates a rod structure of appropriate diameter. Preliminary molecular modeling of the aggregation of six molecules of 3h (without solvation) verify the favorable energetics of this molecular organization. The minimized energy of a six molecule aggregate was 79.7kcal/mol while the energy of six noninteracting (ca. 20 A separation) arborol molecules was 305 kcal/mol. The outer surface of these rods is covered by the terminal hydroxy groups which permit the rods to "cross-link" via hydrogen bonding through adjacent water molecules. Organization of the molecules into the cylindrical aggregates occurs at shorter chain lengths for [6]-n-[6] arborols because the smaller spherical size and lipophilic relationship favors the diminished aggregate diameter for maximized interactions cascade spheres are smaller. The gel structure was also visualized by fluorescence microscopy. The hydrophobic dye chlortetracycline (CTC), which is soluble and nonfluorescent in aqueous solution, fluoresces at 520-530 nm (excitation 97%) of these arborols was with 100% water as eluant until the first peak was collected, and then determined by analytical HPLC (Zorbax ODS, detection at 214 nm) by gradient elution with MeCN-H,O (0-50% MeCN, 20 min) gave a secgradient elution with MeCN-H20 (0-100% MeCN, IO min) and by ond fraction. The first fraction contained 5-(hydroxymethyl)-5preparation of the acetate derivative.I0 'H NMR and infrared data, as methyl-2-oxazolidone ( 9 ) : 11 1 mg (97%); mp 109-1 11 OC; IH NMR 6 well as complete "C NMR spectral data for the arborols 5 (Table VI) 1.13 (s, CH3, 3 H), 3.24 (s,CHZOH, 2 H), 3.34 (s, CHIOH, 1 H), 3.83 and their acetate derivatives (Table VI]), are given in the supplementary (d, COZCHaHb, Jp,b = 8.34 HZ, I H), 4.15 (d, CO$H,Hb, J,.b = 8.34 material. Attempted melting point determinations of 5 caused efferHz, 1 H), 7.45 (NH); 'IC NMR 6 22.4 (CCH,), 58.1 (C,,,), 66.4 vescent decomposition at temperatures of 140-180 OC and, thus, no (CH20H), 72.0 (CH202CNH),158.6 (C=O); IR (KBr) 3285, 2976, melting points are reported. 2931,2876, 1743, 1475, 1400, 1302, 1261, 1193, 1046, 1008,969,934, Sa ( n = 1): 66%; 'IC NMR 6 24.1 @ G I 2 ) , 51.5 (a-CH), 60.4 826,771, 71 I cm-I. Anal. Calcd for C5H9NOl: C, 45.80; H, 6.92; N, (CHZOH), 170.4 ( C 4 ) . 5b (/I = 3): 69%; 'IC NMR 6 24.5 (B-CH,), 10.68. Found: C, 45.57; H, 6.93; N, 10.69. 53.5 (n-CH),60.4 (CHZOH), 171.0 (C-0). Sc (/I = 4): 65%; "C The second fraction was N,N"N",N"'-tetrakis[ 1,I-bis(hydroxy53.5 (a-CH),60.6 (CHZOH), 171.1 ( C 4 ) . Sd NMR 6 26.6 (&CY,), methyl)ethyl]-l,1,10,10-decanetetracarbxamide(8): 235 mg (81%); 'H ( n = 5): 72%: "C NMR 6 26.4 (fl-CH,), 53.5 (a-CH), 60.5 (CH20H), NMR 6 1.12 (s, CH3and (cff2)6, 24 H). 1.61 (m, CH2CH. 4 H), 3.02 Je (/I= 6 ) : 'IC NMR 6 26.6 (&CH,).53.6 (a-CH),60.6 171.1 (M). (t. C H , J = 7.1 Hz,2 H), 3.42 (s, CH2OH, 16 H), 4.78 (s, CHIOH. 8 (CHzOH), 171.2 ( C 4 ) . 5f ( D = 7): 60%; "C NMR 6 26.6 (@-CH,), H), 7.45 (s, NH, 4 H); I3C NMR 6 18.4 (CHI), 26.7 (C-2 and C-9), 28.7 53.7 (a-CH),60.4 (CH,OH), 171.1 (C-0). 5g ( n = 8): 86%; 'IC and 28.8 (C-4, C-5, C-6, and C-7), 30.4 (C-3 and C-8). 53.9 (CH), 58.2 60.6 (CH2OH). 171.2 ( C 4 ) . 5h NMR 6 26.6 (@-CH2),53.4 (@-Of), (C,,,,), 63.7 (CH20H),170.4 (C=O).Anal. Calcd for C30HSBN4012: ( n = 9 ) : 84%: "C NMR 6 26.6 (@-CH2),53.6 (a-CH),60.6 (CHZOH), C, 54.04; H, 8.77; N, 8.40. Found: C, 53.84; H, 8.68; N, 8.20. 171.2 (C=O). 5i ( n = 10): 62%; IIC NMR 6 26.7 (,9-CH2),53.8 Microscopy. The [9]-IO-[9] arborol (3h) (20 mg) was dissolved in (a-CH),60.5 (CH20H), 171.3 (C=O). 5j ( n = 11): 69%; I1C NMR deionized distilled water ( I mL) at 80 OC, then allowed to gel at 25 OC. 6 26.7 (P-CH,), 53.7 (a-CH). 60.5 (CHZOH), 171.3 (CEO). 5k ( n = A portion of the gel was transferred onto a parlodion support film on a 12): 61%; "C NMR 6 26.6 (P-CH2). 53.7 (a-CH). 60.5 (CHZOH), 50-mesh copper EM grid (Polaron Instruments Inc.) and air-dried for 171.2 (C=O). 51 ( n = 13): 68%; "C NMR 6 26.6 (,9-CH2),53.7 1 h. The gel was stained by addition of a drop of 2% aqueous phos(a-CH),60.8 (CHZOH), 171.2 (CEO). photungstic acid (pH = 6.8); excess solution was drawn off with filter N,N'-Bis(2-hydroxy-l,l-bis(hydroxymethyl)ethyl]-a,w-alkanedipaper, and the grid was air-dried for 1 h. Grids were viewed on a JEOL carboxamide ( 6 2 ~ ) General . Amide Formation. A mixture of Tris (16.0 100-CX TEM at an accelerating voltage of 80 kV. Size comparisons
J . Am. Chem. SOC.1990, 1 I2, 8465-8472 were made by using tobacco mosaic virus (diameter = 180 A). as a reference. Arborol3h (20 mg) was dissolved in deionized water (1 mL) at 80 "C, allowed to gel at 25 "C, and then lyophilized for 24 h. A portion of the dried gel was transferred into a drop of water on a microscope slide. The reconstituted gel was viewed on a Leitz Ortholux I I microscope equipped with polarized light optics. A A-plate and A/4-plate were inserted in the light path for color contrast; photographs were taken on Ektachrome film (Kodak) and are reproduced in black and white. Viscometry. The solution was transfered into a Wells-Brookfield cone and plate, stcady shear viscometer (Model LVDCP). Shear rates were adjusted from 450 to 2.25 Hz. GetSoltition Phase Transition. Aqueous solutions of 3h (2.1 2, 4.34, 6.22, and 8. I5 wt 3' %) were prepared with use of water from a three-stage Millipore R/Q water purifier. The gels were heated above the phase transition (ca. 80 "C) filtered through Durapore 0.22-pm filters into precleaned fluorimeter cells. Portions of these samples were used for polarized light microscopy. Gel melting points were determined by the disappearance of birefringence between crossed polars on an Olympus BH-2 microscope using a Mettler FP800 thermally controlled stage, increasing the temperature 2 "C/min. The phase-transition temperatures were also observed by light scattering experiments performed at a scattering angle of 90". The instrument consisted of a Hughes helium-neon laser, Pacific Precision Instruments Model 126 photon counting system, and Hamamatsu R928P photomultiplier. Temperature regulation was accomplished either by a Lauda RCS-6 bath or an Omega CN-2010 electrical temperature controller using a temperature ramp of 2 OC/min. Phase-transition temperatures were taken as a decrease of the scattered intensity to 10% of its initial value. pH Stability of Gels. The pH of the solutions was measured with a Corning Modcl 7 pH meter with a combination electrode. The arborol
8465
(20 mg) was added to the solution (0.40 mL) and the mixture warmed until a homogeneous solution was obtained (ca. 80-90 "C). After solution was cooled to 25 "C, the Occurrence of gel formation was noted. For data, see Table I I . Molecular Modeling. Calculations were performed on a Silicon Graphics IRIS 4D/50GT superworkstation with use of Polygen's Q u A N T A / c H A R M m software.6 Preliminary structures were input via CHEMNOTE and minimized by using initially the conjugate gradient and then the adopted-basis Newton-Raphson methods until the RMS deviation was
